An electro-photographic type image forming apparatus known in the art includes a fixing device in which transferred toner image is melted by heating to fix the toner to a sheet. The fixing device includes a heater such as a halogen heater and a ceramic heater for melting the toner. The molten toner is pressed against the sheet and is fixed to the sheet. Such type of fixing device becomes widespread because of high fixing speed and high imaging quality. However, the image forming apparatus provided with such fixing device is unsuitable in terms of electric power saving because large amount of electric power is required for heating the toner.
In order to solve the above-described problem, there exists an image forming apparatus provided with a fixing device in which a fixing solution for dissolving or swelling toner is applied onto a toner on the sheet to fix the toner image to the sheet. According to such fixing device, heating treatment for melting the toner is not required contrary to the thermal fixing type, providing low electricity consumption which leads to power saving. The fixing solution is coated on a surface of a roller, and the fixing solution on the roller is brought into contact with the toner image for coating the fixing solution onto the toner image.
According to a fixing device described in Patent Document 1, physaliform fixing solution is held on a fixing roller, and the physaliform fixing solution is coated on the toner image, so that the fixing solution melts the toner for fixing the toner image.
According to another method for coating fixing solution onto the toner image, the fixing solution is coated on the toner image in contactless manner by using a spraying device (see Patent Document 2). The fixing device includes spraying means for spraying the fixing solution toward the sheet, and second charging means to which electric voltage is applied. The fixing solution is coated on the toner image by electrostatic spraying.
According to the fixing device disclosed in Patent Document 2, a heater is provided for adjusting temperature of the fixing solution to maintain the liquid at a constant temperature. Viscosity of the fixing solution is changed dependent on the temperature. Regarding the fixing solution whose viscosity is lowered in accordance with elevation of temperature, supplying amount of the fixing solution is increased in accordance with the temperature elevation, wastefully consuming the fixing solution. This drawback must be overcome.
Patent Document 3 discloses a fixing device in which the fixing solution is sprayed onto a developing agent image. This fixing device includes a shutter movable in a widthwise direction of a recording sheet. The shutter is positioned to shut off the spray of the fixing solution at a position between the recording sheet and a spraying head. The shutter is so controlled that the widthwise position of the shutter is changed in accordance with a type of the recording sheet. This structure prevents the sprayed liquid from being deposited on an article other than the recording sheet.